Ferroelectric perovskite oxides with a spontaneous electric polarization have potential applications in nonvolatile random access memories and high-density data storage devices. Following consumer demands for higher density data storage technologies, a continuing miniaturization of electronic components has been the major focus of researchers and industry. To achieve this aim for devices which integrate ferroelectric oxides, one of the most important issues is how to maintain ferroelectricity (and electric polarizations) when the component size reduces to the nanometer scale. Specifically, finding effective and robust routes to reduce the critical thickness (before ferroelectricity vanishes) and thus improve ferroelectric stability of ultrathin films is of critical importance.